How do political parties shape state capacity? We argue that democratic leaders backed by personalist parties are more likely than other leaders to undermine impartial state administration. Personalist parties are those where the leader has more control over the party than other senior party elites. Elites in these parties have careers closely tied to the leader, are unlikely to normatively value an impersonal bureaucracy, and lack collective action capacity independent from the leader. Therefore, personalist parties are less likely than other parties to restrain leaders from undermining impartial state administration. Results from various designs for causal inference show that party personalism decreases impersonal state administration, particularly when the party controls a legislative majority. However, party personalism does not influence other dimensions of state capacity, such as fiscal capacity or territorial control. The findings have implications for how political parties enable democratically elected leaders to erode open-access societies and ultimately, democracy.